Polymeric foams in particular polyurethane foams are well known. In general their preparation requires the mixing of reactive chemical components, such as a polyol and an isocyanate, in the presence of normally used additives such as a suitable catalyst, a surfactant or cell control agent, and water which chemically reacts with the isocyanate to produce the carbon dioxide for blowing the foam.
In the continuous production of flexible foams and particularly in the production of flexible foams in slab-stocks, as currently practised on conventional machines, it is common practice to spread or pour a thin layer of the mixture in a liquid state onto a moving sheet substrate provided on a slightly sloped conveyor and then the foam is allowed to rise freely, due to reaction between the chemical components, until the total expansion of the foam is obtained. The foam is then allowed to cure and thereafter is cross-sawn into blocks. Convention process and apparatus are described, for example, in U.S. Pat. Nos. 3,325,823 and 4,492.664.
In order to avoid a situation where the liquid mix underruning the foam, and to allow the production of uniform blocks, use of a small slope, and high speed for the conveyor and high chemical out put are usually required. This results in costly and large space consuming machines, as well as in an excessively high production rate and very large scale plants.
In an attempt to partially remedy the problems and disadvantages involved by a conventional process, U.S. Pat. No. 3,786,122 suggests an alternative foaming procedure, in which liquid reactants are mixed and introduced in the liquid state, at the bottom of a pre-foaming trough. This allows the mixture to expand upwardly causing the pre-expanded mixture to flow out of the vessel, on a channel-shaped sheet material travelling on a conveyor device. Although this process eliminates the use of reciprocating mixing heads in the production of a continuous slab of a polymeric foam, nevertheless some problems arise due to "build-up" of the foam in the trough which causes a progressive narrowing or reduction of the useful volume of the trough and consequently a reduction in the residence time in the through. The partly expanded foam is still of a relatively high density and low viscosity and limits the slope angle that can be used for the foaming conveyor. Consequently, the risk of foam underrunning, therefore, still exists.
Although the main object of the U.S. Pat. No. 3,786,122 was to employ a conveyor shorter in length, running at a speed slower than the conveyor in a conventional machine, the fact that the mixture emerging from the vessel is still in a liquid state practically prevents the speed and the length of the conveyor to be reduced to any substantial extent. Therefore the resulting slab-stock foaming machine was large and still required large spaces. A Publication PU Handbook G. HOERTEL Ed. Carl Hauser Verlang--1985 pages 162-168 describes a further variation of the pouring technique in the attempt to achieve an uniform lay-down of a mixture of polyurethane components by a fixed mixhead. This mixture being discussed by Hoertel is not a froth nor does blowing occur. According to Hoertel the mixture is spread onto the whole width of the conveyor by the use of a distribution bar through which the mixture is delivered across a broad front. In the same manner the resulting flow is similar to that resulting from the trough used in the U.S. Pat. No. 3,786,122; depending on the volume of the distribution bar and the chemical reactivity of the foam system, the mixture is delivered onto a substrate as a clear (no reaction) or an already creamy (reaction started) liquid. Consequently, density and viscosity of the mixture still depend on the volume of the distribution bar in which chemical foaming takes place. Therefore, the distribution bar according to Hoertel does not substantially differ from the trough system of U.S. Pat. No. 3,786,122.
The frothing process is a well known technique in polyurethane technology but not in the production of slab stock foam. When frothing, a non-reactive inert gas, or blowing agent, is mixed under pressure in a liquid state or in solution with the polyurethane chemical components in a mixer. The pressure is subsequently released causing the frothing or pre-expansion. The vaporization of the blowing agent causes the cells to grow and to foam the liquid reaction mixture which cures to form an elastomer.
Typical blowing agents are the various chlorofluorocarbons (CFC), however certain environmental problems are associated with the use of CFC materials. Therefore, many attempts have been made to produce foamed polyurethane materials, by frothing with carbon dioxide (CO.sub.2).
Carbon dioxide (CO.sub.2) as non-reactive blowing agent, in the frothing technique, has been suggested for example by U.S. Pat. Nos. 3,184,419 and 5,120,770.
According to these two patents, the reaction mixture is subjected to a pressure during mixing, to maintain the blowing agent in the liquid state. Thereafter the mixture is ejected at atmospheric pressure causing a turbulent vaporisation of the blowing agent. Therefore, while the froth technique and the use of an inert blowing agent incorporated in a liquid state into the reaction mixture, allows the manufacture of a foam of reduced density, nevertheless the cell structure is of very inconsistent quality due to irregular shaped and oversized cells or bubbles being present.
Although frothing with inert gas, in particular CO.sub.2, is a well known potential technique, up to now no successfully practicable frothing process and system have been suggested or discovered for use with and in slab stock foam production.
In an attempt to solve the problem of slab stock production without use of chlorofluorocarbon blowing agents, and by using the frothing techniques, it has been now discovered that the suitable release of the mixture under pressure must take place under controlled conditions. Use of controlled conditions in the production of polymeric foams positively influences the growth of the cells during initial frothing expansion of the mixture, which is of importance in the production of slab stock.
Presently, the need for a new foaming process and system in the continuous manufacturing of flexible slab-stock or other continuous foam production lines, in which the frothing technique and a non-reactive blowing agent could be practically usable, still exists.